To get an idea of the effect of group riding versus solo riding, check out the lap information for
this ride. Click on "Dashboard", then click on "Laps". This was my ride from last Saturday. I rode solo from my house to the start of the ride; a distance of 3.2 miles, at an average speed of 12.9 mph (lap 1). Then, I met up with a group of about 10 and rode with them for 19.7 miles at an average speed of 17.0 mph (lap 2). Then, I split off the group and rode solo back to my house for a distance of 11.7 miles at an average speed of 14.4 mph (lap 3).
If you use the average heart rate as an indicator of how much I was pushing myself, you will see that I was not working any harder going 17.0 mph in the group (average heart rate 126 bpm) as I was riding solo at 14.4 mph (average heart rate 130 bpm).
Never underestimate the power of the group.
It does not matter how fast you can ride to participate in the Thursday rides, as long as you are up to riding for two and one-half hours. One of the disadvantages of starting this time of year is that the days are long. The Thursday rides are designed to go from 6:00 PM until sunset. When we start in March, the days are short, so we only ride for about 1 hr 20 min. Each week, we ride about 5 minutes longer until the end of June. Then we have about four weeks where the day length plateaus, then they start getting shorter again until we quit in September. That is why it is important to start earlier in the year. Still, I am not concerned how slow you might be, as long as you are up to spending 2 1/2 hours in the saddle (actually, 2 hr 28 min for this week, increasing to 2 hr 35 min on June 25 and July 2).